Infrared laser absorption spectroscopy is an extremely effective tool for detecting trace gases. Presently, however, the usefulness of laser spectroscopy is limited by the lack of broadly tunable, mode-hop-free sources in the spectroscopically important mid-IR region, defined herein as wavelengths between ˜3 and 30 μm.
Quantum cascade (QC) and Interband Cascade (IC) lasers are excellent light sources for spectroscopic applications in the mid-IR. The high power of QC and IC lasers permits the use of advanced detection techniques that improve S/N ratio of trace gas spectra and decrease the apparatus size. In addition, the large wavelength coverage available with QC and IC lasers allows numerous molecular trace gas species to be monitored.
Spectroscopic applications require single mode operation, which can be achieved by introducing a distributed feedback (DFB) structure into the QCL active region. Experiments using distributed feedback (QC-DFB) lasers have demonstrated the efficacy of these devices for sensitive, highly selective real time trace gas concentration measurements based on absorption spectroscopy, with sensitivities of several parts per billion (See e.g. K. Namjou et al., “Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser,” Optics Letters, V. 23, n. 3, Feb. 1, 1998, which is hereby incorporated by reference).
Although QC-DFB lasers show high performance and reliability, they are useful only over narrow wavelength ranges. This is because the range of wavelength tuning of the emitted laser radiation is limited by the tuning range of the DFB structures. Typically the maximum tuning range of DFB-QCLs is of ˜10 cm−1 and is achieved by varying either the temperature of the chip or the laser injection current. One of the disadvantages of thermal tuning is that it affects the effective gain of the QCL, which in turn causes the output laser power to decrease with increasing temperature of the QCL chip.
Thus, to take full advantage of the wavelength tunability potential of a QCL, an external cavity (EC) configuration must be applied. However, high quality AR coatings, which are necessary for mode-hop-free EC laser operation are not available for the mid-IR spectrum. The lack of effective anti-reflective coatings in the mid-IR range means that it is impossible to achieve tuning across the wavelengths within the gain curve without experiencing mode-hopping. When mode-hopping occurs, the laser changes its frequency discontinuously. A laser that exhibits discontinuous tuning is not useful in high resolution spectroscopic applications such as spectral measurements of ro-vibrational molecular transitions.
One known approach to avoiding mode-hopping is to change the external cavity length synchronously with the grating angle, which is usually realized by appropriate selection of the grating pivot point. This approach works when an effective AR coating is available, such as in the visible and near-IR spectral regions, but cannot be used in the absence of an effective AR coating, such as in the mid-IR range.
Hence, it is desirable to provide a widely tunable, mode-hop-free external cavity laser that is functional in the mid-IR wavelengths.